The blades of a wind turbine experience loads during operation due to aerodynamic, gravitational and inertial forces. Excessive loads can damage the blades and also other components of the wind turbine, such as the generator drive train, for example, and so it is important to ensure that the loads experienced by the blades remain within acceptable limits. In order to monitor this, modern wind turbines are usually provided with a blade load sensor system. Such a system typically includes a load sensor provided in each blade to measure the load experienced by that blade. The load sensors are usually provided in the form of a strain gauge and more particularly as an optical strain gauge, for example similar to the one described in EP1230531.
All strain gauges are fundamentally sensitive to temperature due to thermal expansion of the measured object (i.e. the blade) being detected as strain by the gauge, or due to direct temperature effects on the sensor package itself. To accommodate this, blade load sensor packages are known that include a temperature sensor so that the load signal can be compensated for temperature effects using data from the temperature sensor.
However, if the temperature sensors malfunction, the accuracy of the measured load values will be adversely affected. It is therefore desirable to provide a blade load sensor system that provides reliable load measurements even if the temperature sensors are malfunctioning.